Our focus in this paper is on theories which are Carrollian instead of Poincare invariant. The wonderfully named Carroll group (after Lewis Carroll of Alice's Adventures in Wonderland fame) is obtained by a priori a rather bizarre contraction of the Poincare group, where the speed of light goes to zero [1]}. Interestingly, these theories encompass theories living on null hypersurfaces. Two of the most important classes of null hypersurfaces that we encounter in physics are the null boundary of asymptotically flat spacetimes and the event horizons of generic black holes. If we are interested in defining field theories that live on null surfaces, it is of interest that the degrees of freedom of the field theory don't leave this null surface at a later time. Any massive degree of freedom cannot travel at the speed of light and hence would not do the job. So it is natural to consider massless Carrollian theories, which are tantamount to conformal Carrollian theories.
